Yeh, there's another one I'd like to get at and change tho. From the manual:
[quote:2a9h8une]
Tiled Background expressions
ImageWidth
ImageHeight
The original dimensions of the tiled background's current image in pixels. Since tiled backgrounds can be extended over large areas causing the normal Width and Height expressions to return different values, these can be used to get the original size of the source image regardless of the object size.